(1) Field of the Invention
The present invention relates to circuit boards which are suitable for use at microwave frequencies. More specifically, this invention is directed to circuit boards and a method of forming circuit boards having through-hole connections in a laminate which includes a plastic film sandwiched between copper and aluminum layers.
(2) Description of the Prior Art
The present invention has particular utility in the formation of circuit boards for transmission of electrical signals at microwave frequencies. Circuit boards for use at microwave frequencies often have a comparatively thick aluminum plate or layer which, among other uses, functions as a heat sink. A layer of plastic material, selected for its dielectric and mechanical properties, will be adhesively bonded to one surface of the aluminum layer and the circuit board laminate will also have a layer of copper foil bonded to the exposed surface of the dielectric film, i.e., the plastic layer. The dielectric film may, for example, be comprised of polytetrafluoroethylene (PTFE) or fiber reinforced PTFE. This laminate of dielectric material sandwiched between a relatively thick lower layer of aluminum and a relatively thin upper layer of copper foil may be viewed as a preliminary, starter, or intermediate circuit board structure.
In such circuit boards it is necessary to establish electrical connections between portions of the circuit, which will be formed on or in the copper foil layer, and the aluminum plate which forms part of the ground plane. These connections are desirably implemented via through-holes in the laminate. The establishment of such through-hole connections in a reliable and cost-effective manner is a problem of long standing in the art. Most desirably, the through-hole connections will be effected by electroless plating of copper. However, it is not possible to produce, simply by electroless plating, a conductive path between spaced copper and aluminum elements. It is, in fact, well known in the art that plating upon aluminum requires extraordinary treatments to obtain the requisite adhesion.
There are presently two principal techniques for plating upon aluminum. These two techniques are the zincating process, wherein a thin layer of zinc is deposited upon the aluminum base and other metals are subsequently deposited over the zinc, and the anodizing process where a porous anodic coating is produced and a metal subsequently plated over the coating. Because of its comparative ease of implementation and relatively low cost, the zincating process is by far the more widely use technique. However, this technique has the disadvantages that it is very sensitive to the condition of the surface of the aluminum thus requiring time consuming pre-zincating cleaning and etching treatments. Further, it is generally considered inadvisable to use zinc coated aluminum members in electrical circuit applications since the zinc will melt during soldering, mix with the solder and produce high resistance or otherwise defective connections. The foregoing is also true of cadmium which is sometimes substituted for zinc.
Anodizing processes have not been employed in the production of microwave circuit boards since it has been universally believed that the copper would prevent an anodic coating from forming on the aluminum surface or the copper would be chemically attacked and thereby rendered unfit for use.
Accordingly, when faced in the prior art with the necessity of establishing a through-hole connection between the aluminum and copper layers of a microwave circuit board laminate, and being unable to employ a zincating process, resort has been had to the unreliable technique of trying to deposit copper or nickel either directly on the aluminum or on aluminum which has previously been given a chromate conversion coating. As is well known, such methods produce poor quality plated metal coatings that exhibit low bond strength to the aluminum base metal. Thus, exposure of these poor quality coatings to elevated temperatures, for example soaking for one hour at 250.degree. F. or immersion in molten 60/40 tin/lead solder, will cause the coating to blister.